Eyewitness Steve Webster saw the crash. He thought the helicopter was in difficulties before it hit the crane.
Nicola Morgan MP described to ITV News how she heard an "almighty bang" and saw "clouds of thick black smoke" as she walked towards Vauxhall tube station.
Michael Gavin saw the accident while he was standing at Vauxhall station.
– Michael Gavin, speaking to BBC News
I was facing the building when it happened and I heard a loud bang. The top of the crane was actually obscured by fog so I didn't see the impact.
But I heard a bang and saw the body of the helicopter falling to the ground along with pieces of the crane and then a large plume of smoke afterwards.
Charred remains of a car it hit on the ground http://t.co/DC9OAMeH
What looks like tail wing section of helicopter on roof of building http://t.co/5e5iF6aY
Chris Matthison was near to the crash site.
– Chris Matthison
I heard a very unusual dull thud, then there was silence. The silence really took my imagination. Emergency services responded very quickly.
There was some damage to the crane. It's possible the crane is lying across Nine Elms road. The top of the nearest building is steeped in mist and difficult to see.
Sarah-Beth Casey, who had three small children with her in her apartment, saw the immediate aftermath of the helicopter crash.
– Sarah-Beth Casey, speaking to Sky News
When I heard the explosion - it was like a little earthquake. It was like a gas explosion. I looked up to see debris falling off the tower. I can't see any people on the ground, it seems to be a lucky escape. The police and ambulance response was incredible.
Erin Rogers was waiting at a bus stop outside Vauxhall station when the crash happened.
– Erin Rogers
It was a bit surreal actually. I just had a coffee in my hand, I looked up, heard a bang and saw bits of crane debris falling to the floor.
Then the helicopter was in flames. The rest of the people at the bus station were looking on going 'What was that?'. It's something I will never forget for a long time.
Paul Ferguson, who was working in a nearby office, saw this morning's helicopter crash.
– Paul Ferguson, speaking to BBC News
There was a flash and the helicopter plunged to the ground. It exploded and you can imagine the smoke coming out of it.
It was probably heading from the nearby heliport. It may be that on this misty morning the lights on nearby St George's Tower weren't on and it moved and clipped the edge of the crane and lost control.
Helicopter was banking right as I came over Chelsea Bridge and v low. Battersea Heliport other side of the river so seemed v odd.
Spotted mayhem near Vauxhall - LOADS of police cars and fire engines. Turns out a helicopter hit a crane. Scary stuff and seen awful pics.
I'm pretty sure all of londons police, fire crews & paramedics are in vauxhall right now. Biiiig fire, hope everyones ok.